Soldering is a low temperature means for mechanically and electrically joining metal or metallized objects. A molten metal alloy, selected for its low melting point, forms the joint.
In microelectronic packaging, soldering is one of the last steps in assembling the package. Discrete components are constructed with pins, leads or surface metal pads for mounting. The substrate is prepared with complementary receptacles or surface metallization to receive the components. Ultimately, the components are soldered to the substrate. Conventional eutectic and pseudo-eutectic metal alloys and mixtures are used in the solder pastes. These pastes have low melting points, usually on the order of 100.degree.-200.degree. C., allowing the peak soldering temperature to be as low as 120.degree.-220.degree. C.
The low temperatures of soldering render it particularly advantageous for microelectronic package applications. Thermal stress is minimal on the parts being joined, and the integrity of these parts, especially the lower melting circuitry patterns therein, is not compromised by the subsequent low temperature solder operation.
But, soldering joints also have disadvantages. Most notably, the solder joint can be damaged during subsequent high temperature processing or use in a thermally hostile environment, such as under the hood of an automobile. As the temperature approaches the melting point of the solder metal, it softens and flows. Metal in the solder starts to diffuse undesirably into surrounding metallizations. The mechanical and electrical connection can be completely ruptured. And, when the solder is applied to a metallization on a ceramic surface, the solder attacks the metallization-ceramic interface causing the metallization to pull away from the ceramic.
Endeavoring to overcome the disadvantages of conventional solders, modified compositions have been proposed. In Metallurgica et Materialia, Vol. 25, pp. 2323-2328, 1991, Betrabet et al. disclose a pseudo-eutectic solder paste additionally containing particles of the intermetallic compound Ni.sub.3 Sn.sub.4. Similarly, at the National Electronic Packaging and Production Conference (NEPCON) West 92, Anaheim, Calif., 1992, Clough et al. disclosed a eutectic solder paste reinforced with particles of Cu and the intermetallic compound Cu.sub.6 Sn.sub.5. Mixed results were reported: Betrabet et al. found the Ni.sub.3 Sn.sub.4 intermetallic promising whereas Clough et al. found no appreciable advantage with the reinforced paste composition. A need still exists for a solder paste composition which reliably performs under hostile temperature conditions over an extended period of time.